Many chemical reactions are thermodynamically favorable, but do not occur at useful rates in the absence of substances which catalyze the reaction. By lowering the activation energy, such catalysts can increase the rate of a particular chemical reaction by several orders of magnitude. Applied in a commercial setting, catalysts can significantly reduce the costs associated with electricity-producing devices such as fuel cells.
Fuel cells are electrochemical devices that convert the chemical energy stored in fuels and oxygen into electricity and small molecule byproducts such as water and CO.sub.2. Like conventional batteries, fuel cells possess both an anode and a cathode. Also like conventional batteries, fuel is oxidized and electrons are produced in the anode and in the cathode. An electrolyte connects the anode and cathode. Ionic current through the electrolyte completes the electrical circuit. A significant advantage of fuel cells over conventional batteries is that the former do not require recharging. Rather, fuel cells produce electricity by consuming a fuel such as hydrogen, reformate gas, or methanol.
Thus, machines powered by fuel cells offer flexibility not available in battery-powered machines. For example, like conventional internal combustion engine-powered motor vehicles, fuel cell-powered motor vehicles offer a long range of travel between refueling. Moreover, fuel cell-powered vehicles can be quickly "refueled" at conventional service stations. In comparison, conventional battery-powered vehicles suffer short travel ranges, a problem compounded by the relatively time-consuming recharging process.
Another advantage of fuel cells is that they are capable of providing energy in an efficient manner with relatively few adverse effects on the environment. For example, methanol fuel cells are attractive alternatives to internal combustion engines for mobile applications as they operate at lower noise levels, offer greater energy efficiency, and emit fewer pollutants and greenhouse gases. Likewise, hydrogen fuels cells or reformate gas fuel cells are also quiet, energy efficient, and less polluting than conventional internal combustion engines.
Despite their apparent advantages, fuel cells have not achieved widespread commercial application, in part because the materials used to catalyze the fuel cell reaction have not been optimized. For example, the materials conventionally used as catalysts suffer rapid poisoning (i.e., loss of catalytic activity caused by the fuel or byproducts of the reaction) and poor efficiency (i.e., high overpotentials are required to produce current). Thus, there exists a great need for new electrocatalytic materials that resist poisoning and catalyze the electricity-producing reaction efficiently.
Materials customarily used as anode or cathode electrocatalysts are pure metals or simple alloys (e.g., Pt, Pt/Ru, Pt/Ni) supported on high surface area carbon. For example, the state-of-the-art anode catalysts for hydrocarbon (e.g., methanol) fuel cells are based on platinum (Pt)-ruthenium (Ru) alloys. Heretofore, the best known catalyst was Pt.sub.50 /Ru.sub.50 (numbers in subscript indicate atomic ratios). Gasteiger et al., J Phys. Chem., 98:617, 1994; Watanabe et al., J Electroanal. Chem., 229:395, 1987. Other binary alloys useful as anode catalysts include Pt/Sn, Pt/Mo, Pt/Os, and Pt/Re. More recently, ternary alloys of Pt/Ru/Os have been developed for use as anode catalysts. Ley et al., J Electrochem. Soc., 144:1543, 1997; U.S. Patent No. 5,856,036. The benchmark cathode catalyst for methanol, hydrogen, and reformate gas fuel cells is pure elemental platinum supported on carbon (Pt/C) at a metal loading of 20-30% by weight. Still other materials for use as catalysts are known.
U.S. Pat. No. 4,880,711 to Luczak et al. teaches a ternary alloy catalyst for fuel cells comprising platinum and gallium, and additionally chromium, cobalt, nickel, and/or mixtures thereof. This alloy catalyst requires at least about 50% platinum to be an effective catalytic material. Other elements in the same periodic group, namely iridium, rhodium, osmium, and ruthenium, are indicated to be substitutable for a portion of the platinum.
U.S. Pat. No. 4,127,468 to Alfenaar et al. discloses a process for producing metal electrodes in which a basis-metal electrode comprising a basis-metal which is present in a finely divided or porous state, and which is selected from the group consisting of the noble metals from Groups IB, IIB, or VII of the Periodic Table of the Elements, or an alloy of at least one of said metals, is contacted with a solution containing an alloying element. The alloying element is selected from the group consisting of an element from Groups IIIA, IVA, VA, VIA, VII, IB, IIB, VIIB, or combinations thereof, of the Periodic Table of the Elements. The alloying-element compound is reduced in situ to form a free-alloying element, whereby the alloying element forms an alloy with the basis-metal. Preferred basis-metals include palladium, platinum, palladium-platinum, and platinum-iridium.
U.S. Pat. No. 5,208,207 to Stonehart et al. discloses an electrocatalyst comprising a ternary alloy essentially consisting of platinum-palladium-ruthenium supported on an inorganic support. U.S. Pat. No. 5,225,391 to Stonehart et al. teaches an electrocatalyst comprising a four-element alloy consisting essentially of platinum, nickel, cobalt, and manganese supported on an inorganic support. U.S. Pat. No. 5,286,580 to Ippommatsu et al. discloses a fuel electrode for a high temperature solid electrolyte fuel cell comprising ruthenium, osmium, rhodium or iridium, or an alloy thereof. U.S. Pat. No. 5,876,867 to Itoh et al. discloses an electrocatalyst comprising an alloy of platinum with a base metal selected from gallium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper, the alloy being supported on a conductive carbon powder, the electrocatalyst having a structure of vacant lattice site type lattice defects.